Terrestrial broadcast has heretofore been transmitting amplitude-modulated signals, and CATV has also been transmitting amplitude-modulated signals. In the United States, it is expected that ATV (Advanced Television) system broadcast will start in 1998 as terrestrial digital broadcast.
Concerning the modulation system, a terrestrial broadcast is already in service which transmits digital data which has been 8VSB-modulated. In CATV industry, a service is being planned to transmit digital data which has been 16VSB-modulated. Some CATV operator has already started to provide a service to transmit QAM-modulated digital data.
In this way, there exist not only difference of analog and digital broadcast systems but also broadcast employing different modulation schemes intermingled making increasingly more types of broadcast available for reception at home.
As a practical example, there is broadcast of analog VSB-modulated signals as employed in the current NTSC system broadcast. A broadcast using VSB-modulated signals has a video carrier 19, an audio carrier 17, and a color sub-carrier 18 of which the frequency relationship is as shown in FIG. 15. In a typical television receiver, as shown in FIG. 16, analog VSB-modulated signals undergo frequency conversion to an intermediate frequency (IF) via a built-in analog VSB tuner 2, while the video band (containing a video carrier and a color sub-carrier) undergoes band limiting by means of a band limiting filter 4 such as a SAW filter and the like having a frequency characteristic 20 as shown in FIG. 15, followed by conversion to base-band signals by means of an AM detector 13 and video processing by means of an analog VSB video processor 9. In the audio band, the audio carrier undergoes band limiting by means of a band limiting filter 6 such as a SAW filter and the like having a frequency characteristic 16 as shown in FIG. 15, followed by FM detection and audio processing of the audio signals by means of an analog VSB signal processor 7.
In the AM detector 13 the construction of which is as shown in FIG. 18, by feeding into a mixer 28 an IF signal 23 obtained by frequency conversion by means of the analog VSB tuner 2 and a local oscillation signal from a local oscillator 30, a Quadrature signal 29, which is the difference between the IF signal 23 and the local oscillation signal, is obtained as an output from the mixer 28. The signal 29 is subsequently allowed to pass through a low-pass filter (LPF) 31 for conversion into an auto phase control (APC) voltage 32, which is then fed back to the local oscillator 30 of the AM detector to correct deviation in phase.
In the ATV system planned as a terrestrial digital broadcast, the terrestrial broadcast signal after digital VSB modulation is a modulated wave having a root raised cosine characteristic with a roll-off rate of 5.7% as shown in FIG. 17. Also, its construction from the tuner to the digital VSB detection is as shown in FIG. 19 according to a ATSC (advanced Television System Committee) Digital Television standard. A double super heterodyne tuner is employed as a tuner 42. VSB detection is done by first feeding to a mixer 25 an IF signal as frequency-converted by the tuner 24 and a signal from a local oscillator 30 after shifting the phase of the local oscillation frequency by 90 degrees, then feeding the Inphase signal 26 from the output of the mixer 25 to an LPF 34 and a limiting amplifier 35. On the other hand, the IF signal and the local oscillation signal from the local oscillator 30 are fed to a mixer 28 to obtain a Quadrature signal 29. By feeding the output of the limiting amplifier 35 and the Quadrature signal 29 to a mixer 36, an S-shaped signal as shown in FIG. 20 is obtained as an output from the mixer 36. This signal is then allowed to pass an LPF 33 for conversion to an APC voltage 32, which is fed back to a second local oscillator of the tuner 42 to correct deviation in phase.
FIG. 24 shows the construction of a broadcast receiving apparatus for VSB-modulated broadcast reception by taking as an example construction of a broadcast receiving apparatus down to the demodulator section of 8VSB-modulated signals for digital terrestrial broadcast and for 16VSB-modulated signals of digital CATV broadcast of the United States. The received signal is frequency-converted by a tuner 97, bandwidth limited by a band limiting filter 98, and detected by a detector 100, then the analog data is converted to digital data by an AD converter 101. Subsequent section is a digital processing section, where the AD-converted digital signal is fed to a synchronous detection circuit 102 to detect segment synchronous signal existent in the ATV format of the Unites States and field synchronous signal to a waveform equalizer circuit 103 to eliminate ghost as well as to correct waveform distortion, to a phase noise reduction circuit 104 to reduce the phase noise component produced in the tuner and others, and finally to an error correction circuit 106 to correct any data error, before inputting to a transport decoder 107 as a transport stream.
There exists another digital broadcast system, namely, CATV broadcast using QAM modulation. The construction down to a demodulator section of a QAM-modulated signal receiver to receive QAM-modulated digital CATV signal is as shown in FIG. 25. QAM-modulated signal is frequency-converted by a tuner 109, bandwidth limited by a band limiting filter 110, Inphase and Quadrature data is extracted by an orthogonal detector 112, and analog data is converted to digital data by an AD converter 113. The digital Inphase data and Quadrature data are fed to a carrier regeneration circuit 114 to correct frequency and phase, to waveform equalization circuits 115 and 116 to correct waveform distortion of the Inphase data and the Quadrature data, respectively, to a phase noise reduction circuit 117 to reduce phase noise component produced in the tuner and others, to a demapper circuit 119 to covert the Inphase and Quadrature data to serial data, and to an error correction circuit 120 to correct any data error, before inputting to a transport decoder 121 as a transport stream.
Since many broadcast signals having difference of either analog or digital and of modulation schemes are transmitted at the same time, it is necessary to prepare separate broadcast receiving apparatus to receive respective broadcast.
As a practical situation, it is likely that analog VSB-modulated terrestrial broadcast which can transmit one channel using one carrier wave, and digital VSB-modulated terrestrial broadcast which can transmit multiple channels using one carrier wave will come into wide use requiring separate broadcast receiving apparatus in order to receive analog VSB-modulated terrestrial broadcast and digital VSB-modulated terrestrial broadcast. In this case, it will be necessary either to prepare separate broadcast receiving apparatus or to prepare a broadcast receiving apparatus which can receive both analog VSB-modulated terrestrial broadcast and digital VSB-modulated terrestrial broadcast. However, if one tries to simply integrate functions of both receivers the cost would become very high.
In an area where digital terrestrial broadcast employs 8VSB modulation whereas digital CATV broadcast employs QAM modulation, in view of the economy and sharing of the receiver,
it may be necessary to receive 8VSB-modulated signals of digital terrestrial broadcast and QAM-modulated signals of digital CATV broadcast with a single receiver. In this case, the circuit down to the demodulator section of a receiver for digital terrestrial broadcast and that of a receiver for digital CATV broadcast will require separate circuits because of the difference of modulation schemes in the 8VSB modulation, 16VSB modulation and QAM modulation. Consequently, if one wishes to receive both the 8VSB-modulated signals of digital terrestrial broadcast and QAM-modulated signals of digital CATV, there will arise a problem of increase in the receiver cost and the number of receivers required by a user.